"Is there any freight on this line?"
There's minimal local freight service from Waterbury as far south as Beacon Falls run by Springfield Terminal (Guilford) once or twice a week, plus Providence & Worcester uses the portion of the line between Devon and Derby Junction for stone trains during the warm weather months.
As a former longtime resident of the Naugatuck Valley, I *never* saw any attempt at advertising rail service, and I too wonder how people were supposed to know about it. It's probably true that a lot of people don't know it exists. Plus, given the infrequent schedule, it's often more convenient to drive to a mainline station like Milford, Stratford, or Bridgeport and take the train to New York from there.